From d63ac118e817ecab6fd4f890960f4f73b4dfd5e8 Mon Sep 17 00:00:00 2001
From: Robert Morris <rtm@csail.mit.edu>
Date: Thu, 8 Sep 2016 14:22:38 -0400
Subject: [PATCH 1/2] this might fix the reported deadlock, though I can't
 reproduce it.

---
 proc.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/proc.c b/proc.c
index 9e9552936c..eee79af7d2 100644
--- a/proc.c
+++ b/proc.c
@@ -82,6 +82,12 @@ userinit(void)
   acquire(&ptable.lock);
 
   p = allocproc();
+
+  // release the lock in case namei() sleeps.
+  // the lock isn't needed because no other
+  // thread will look at an EMBRYO proc.
+  release(&ptable.lock);
+  
   initproc = p;
   if((p->pgdir = setupkvm()) == 0)
     panic("userinit: out of memory?");
@@ -99,6 +105,12 @@ userinit(void)
   safestrcpy(p->name, "initcode", sizeof(p->name));
   p->cwd = namei("/");
 
+  // this assignment to p->state lets other cores
+  // run this process. the acquire forces the above
+  // writes to be visible, and the lock is also needed
+  // because the assignment might not be atomic.
+  acquire(&ptable.lock);
+
   p->state = RUNNABLE;
 
   release(&ptable.lock);
@@ -141,6 +153,8 @@ fork(void)
     return -1;
   }
 
+  release(&ptable.lock);
+
   // Copy process state from p.
   if((np->pgdir = copyuvm(proc->pgdir, proc->sz)) == 0){
     kfree(np->kstack);
@@ -165,6 +179,8 @@ fork(void)
 
   pid = np->pid;
 
+  acquire(&ptable.lock);
+
   np->state = RUNNABLE;
 
   release(&ptable.lock);
@@ -227,7 +243,7 @@ wait(void)
 
   acquire(&ptable.lock);
   for(;;){
-    // Scan through table looking for zombie children.
+    // Scan through table looking for exited children.
     havekids = 0;
     for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
       if(p->parent != proc)

From 34c2efc1d063f4b366c1017c174d117cc96eb990 Mon Sep 17 00:00:00 2001
From: Robert Morris <rtm@csail.mit.edu>
Date: Thu, 8 Sep 2016 14:45:20 -0400
Subject: [PATCH 2/2] use asm() for lock release, not a C assignment

---
 spinlock.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/spinlock.c b/spinlock.c
index bf863ef104..7b372ef3f1 100644
--- a/spinlock.c
+++ b/spinlock.c
@@ -59,8 +59,10 @@ release(struct spinlock *lk)
   // stores; __sync_synchronize() tells them both to not re-order.
   __sync_synchronize();
 
-  // Release the lock.
-  lk->locked = 0;
+  // Release the lock, equivalent to lk->locked = 0.
+  // This code can't use a C assignment, since it might
+  // not be atomic.
+  asm volatile("movl $0, %0" : "+m" (lk->locked) : );
 
   popcli();
 }